The present invention relates to processes of hydrotreating heavy hydrocarbon oil. More specifically, the present invention relates to a process of hydrotreating capable of producing a low sulfur heavy fuel oil with a sulfur content of 1 percent by mass or less at a lower cost, from a feedstock consisting of a heavy hydrocarbon oil derived from a heavy crude oil with an API degree of 30 or below, without lowering the degree of demetallization.
A process of producing a low sulfur heavy fuel oil is known, in which to pass a feedstock consisting of a heavy hydrocarbon oil containing more sulfur and metal components, through a fixed bed reactor filled with two types of catalysts, i.e., a demetallization catalyst and a desulfurization catalyst so as to reduce the sulfur content of the feedstock.
When a heavy hydrocarbon oil (feedstock) is brought into contact with a hydrotreating catalyst, a desulfurization reaction occurs thereby removing sulfur components in the feedstock, i.e., removing sulfur components from organic sulfur compounds such as benzothiophenes, dibenzothiophenes, mercaptans, thioethers, and dithioethers. In addition to the desulfurization reaction, there occur a demetallization reaction removing metal components such as nickel, vanadium, iron, and sodium, a cracking reaction, a denitrification reaction at the same time. However, as these reactions proceed, they involve the secondary formations of cokes and metals which deposit in the pores or on the surface, of the hydrotreating catalyst. These deposits poison the active sites on the hydrotreating catalyst and cause a reduction in the catalyst activities such as desulfurization activity. Furthermore, the deposits are built up in and plug the pores of the catalyst and cause a reduction in the catalytic activities. In general, the lower the API degree of a feedstock, the greater the sulfur compounds and metal such as nickel and vanadium contained in the heavy hydrocarbon oil derived from the feedstock, resulting in an enhanced deactivation rate of the catalyst. Therefore, when a heavy hydrocarbon derived from a feedstock with a low API degree is treated, it is necessary to drastically reduce the throughput. The molecules containing metals in the heavy hydrocarbon oil are large in size and poor in reactivity and thus lower the degree of demetallization and adversely affect the subsequent devices.
Low sulfur heavy fuel oils are used for electric power, boilers, ships, and industrial furnaces. When an attempt is made to collect a low sulfur heavy fuel oil from a feedstock consisting of a heavy hydrocarbon oil derived from a heavy crude oil containing sulfur and metal components in amounts larger than those of ordinary crude oil and having an API degree of 30 or below by the conventional processes, it requires higher reaction temperature and is accompanied with a remarkably enhanced deactivation rate of the catalyst, resulting in a remarkably shortened life time thereof. Therefore, it was regarded substantially impossible to produce a low sulfur heavy fuel with a sulfur content of 1 percent by mass or less, by the conventional processes. As the result, there arose a problem that heavy crude oils with a low API degree can not be utilized effectively.